1. Field of the Invention
The present invention generally relates to the production of devices that convert chemical energy into electrical energy. More particularly, the present invention relates to pad printing processes for coating an electrode active reagent solution or suspension on a conductive substrate. Preferably, the reagent solution or suspension is of a cathode active material, such as of a ruthenium-containing compound, for an electrolytic capacitor. The ruthenium-containing compound is provided as a printable ink comprising an aqueous or non-aqueous carrier, and a binder, preferably a poly(alkylene) carbonate binder. The present invention also relates to using poly(alkylene) carbonates as a binder in a pressed valve metal anode for an electrolytic capacitor.
2. Prior Art
Electrodes with high specific surface areas result in specific capacitance in the hundreds of μF/cm2. Such electrodes are then appropriate when used as the anode and/or cathode in an electrochemical capacitor and as the cathode in an electrolytic capacitor, which require high specific capacitances.
An anode or cathode in an electrochemical capacitor or the cathode in an electrolytic capacitor generally includes a substrate of a conductive metal, such as titanium or tantalum, provided with a pseudocapacitive oxide coating, nitride coating, carbon nitride coating, or carbide coating. In the case of a ruthenium oxide cathode, the active material is formed on the substrate by coating a suspension or dissolved solution of ruthenium oxide or a precursor thereof, such as ruthenium chloride or ruthenium nitrosyl nitrate. The thusly-coated substrate is then heated to a temperature sufficient to evaporate the solvent and, if applicable, convert the precursor, to provide a highly porous, high surface area pseudocapacitive film of ruthenium oxide on the substrate.
The prior art describes various methods of contacting the substrate with the pseudocapacitive reagent solution. For example, Shah et al. and Muffoletto et al. in U.S. Pat. Nos. 5,894,403, 5,920,455, 5,926,362, 6,224,985, 6,334,879 and 6,468,605, all of which are assigned to the assignee of the present invention and incorporated herein by reference, describe coating a ruthenium-containing reagent solution to a conductive substrate by ultrasonic spraying. Ultrasonic spraying is an improvement over other commonly used techniques including dipping, pressurized air atomization spraying, and deposition of a sol-gel onto the substrate. Capacitance values for electrodes made by these latter techniques are lower in specific capacitance than those made by ultrasonic spraying. It is also exceptionally difficult to accurately control the coating morphology due to the controllability and repeatability of the dipping, pressurized air atomization spraying, and sol-gel deposition techniques, which directly impacts capacitance. While the coating morphology is generally good with an ultrasonically spray deposited coating, this technique has problems with overspray, which impacts production costs, especially when the active material is relatively expensive, such as ruthenium.
Therefore, while ultrasonically spraying an active reagent solution onto a substrate is an improvement in comparison to other known deposition processes that provide capacitors with acceptable energy storage capacities, there is a need to further improve production yields that are negatively impacted by wasteful overspray. Increased production yields result by coating an active reagent solution or suspension onto a conductive substrate using a pad printing technique.